1. Field of Invention
The present invention relates to a flat shield cable. In particular, the invention relates to a flat shield cable that is suitably used for electrical connection to electric equipment etc. of vehicles such as automobiles. The present application claims priority to Japanese Application No. 2002-027078, filed on Feb. 4, 2002.
2. Description of Related Art
In vehicles such as automobiles, shield cables are used for electrical connection to electric equipment etc. FIG. 1 shows a conventional flat shield cable 1 generally having a structure in which a plurality of signal lines 2, each having an insulating cover, and a drain line 3 are twisted together and covered with a shield layer 4, which is covered with an insulating sheath 5. Each signal line 2 is composed of a core conductor 2a and an insulating cover 2b that covers the core conductor 2a. 
Incidentally, the common components of the conventional shield cable 1 of the above kind have the following specifications. The cross-sectional area (hereinafter also referred to as xe2x80x9cconductor sizexe2x80x9d) of each signal line 2 is 0.3 mm2, the outer diameter of the core conductor 2a of each signal line 2 is 1.4 mm, the material of the insulating cover 2b is polyvinyl chloride (PVC), the material of the drain line 3 is copper or aluminum, the material of the shield layer 4 is copper or aluminum, and the material of the insulating sheath 5 is poly(vinly chloride), polyethylene, or the like.
With the above configuration, external noise is interrupted by the shield layer 4 and led to an external ground via the drain line 3, whereby good signals are supplied to various kinds of electric equipment through the signal lines 2.
On the other hand, with the recent rapid development of car navigation and DVD equipment, a monitor, a DVD unit, or the like has come to be installed in automobiles. Such electric/electronic equipment has come to be newly installed in automobiles provides pictures, sounds, etc. with improved resolution or quality. Car navigation monitors, for example, can experience blurring in an enlarged picture of a road map etc. Thus, a certain countermeasure is desired. In particular, a shield cable having better electrical performance, specifically, a better characteristic impedance, is required.
In conventional shield cables, the signal lines 2 and the drain line 3 are twisted together. Therefore, terminal-processing work (i.e., connection to a connector terminal) is difficult to perform and takes time. Further, a terminal processing length (i.e., a length over which the insulating sheath 5 and the shield layer 4 need to be stripped away) of about 80 mm is needed, which causes a problem that the shielding performance deteriorates at a terminal.
The standards for connectors of the electrical/electronic equipment that are newly incorporated in automobiles require that the outer diameter of each signal line 2 be 1.3 mm. However, in the standards for conventional connectors, the outer diameter of each signal line 2 is mainly 1.4 to 1.6 mm (the diameter of a finished signal line is equal to 1.4 mm when the conductor size is 0.3 mm2). As such, conventional connectors do not conform to the equipment that has come to be newly incorporated in automobiles; it is indispensable to reduce the outer diameter of each signal line 2. This results in a problem that a buckle or a disconnection likely occurs in the core conductor 2a of the outside signal line 2 when bending stress is applied to the shield cable 1 in the lateral direction.
An object of the present invention is to solve the above problems in the art and thereby provide a shield cable capable of satisfactorily accommodating electric/electronic equipment that has come to be newly installed in automobiles, maintaining good shielding performance, facilitating terminal-processing work, and effectively preventing a buckle and a disconnection in each signal line even if the outer diameter of each signal line is reduced.
The present inventor has studied intensively to solve the above problems in the art and has completed the present invention by finding out the following. The characteristic impedance of each signal line is strongly related to the problem of the above-mentioned electric/electronic equipment newly installed in automobiles. The impedance has a value of about 50 xcexa9 in conventional cables. When connection is made to such equipment as a DVD unit or a monitor by using a conventional shield cable, impedance mismatch causes reflection between the signal supply side and the equipment side, which is considered the cause of the above problem. It has been found that increasing the characteristic impedance of each signal line to about 75 xcexa9 (xc2x110%) attains impedance matching and can thereby solve the above problem. On the other hand, to satisfy the standards for connectors of the new electric/electronic equipment, the outer diameter of each signal line must be 1.3 mm. A desired characteristic impedance can be obtained by adjusting the cross-sectional area (conductor size) of the core conductor of each signal line and the dielectric constant of the insulating cover of each signal line in consideration of the above factors.
That is, according to the invention, the above object is attained by the following technical means:
(1) A flat shield cable wherein a plurality of signal lines each having an insulating cover are arranged parallel with each other; the plurality of signal lines each having the insulating cover are covered with a shield tape having an impedance adjustment intervening tape and a shield layer in such a matter that the shield layer is located outside; a drain line is provided on one side of an array of the plurality of signal lines each having the insulating cover so as to be parallel with the plurality of signal lines and to be in contact with the shield layer of the shield tape from outside; and all of the plurality of signal lines, the shield tape, and the drain line are covered with an insulating sheath.
(2) The flat shield cable according to item (1), wherein each of the signal lines has an outer diameter of 1.3 mm, a core conductor of each of the signal lines has a cross-sectional area of 0.13 to 0.22 mm2, the insulating cover of each of the signal lines is made of an insulating material having a relative dielectric constant of 1.7 to 2.8, and each of the signal lines has a characteristic impedance of 75 xcexa9 (xc2x110%).
(3) The flat shield cable according to item (1) or (2), wherein the drain line has a cross-sectional area of 0.22 to 0.5 mm2.
(4) The flat shield cable according to any one of items (1) to (3), wherein the insulating cover of each of the signal lines is made of foamed polyethylene.